UPDATE – On April 29, the space station team identified an issue with one of the station’s Main Bus Switching Units that distributes power to two of the eight power channels on the station. There are no immediate concerns for the crew or the station. Teams are working on a plan to robotically replace the failed unit and restore full power to the station system. Additional information will be provided as it becomes available. The earliest possible launch opportunity is no earlier than Friday, May 3.
ORIGINAL STORY – SpaceX stands primed for its fifth launch of the year when the next Dragon cargo mission heads to the International Space Station (ISS) in the small hours of Wednesday morning (1 May). Liftoff of the Upgraded Falcon 9 is scheduled to occur from Space Launch Complex (SLC)-40 at Cape Canaveral Air Force Station, Fla., at 3:59 a.m. EDT.
Launch has been moved back from the original target of 26 April, then 30 April, then slipped to 1 May, due in part to a delayed Static Fire Test of the nine Merlin 1D+ first-stage engines on the Falcon 9, which eventually took place on Saturday. It looks set to be postponed again following an issue with the ISS electrical system, discovered on Monday morning. “There are no immediate concerns for the crew or the station,” NASA noted late Monday. “An issue is being worked with a Main Bus Switching Unit (MBSU) that distributes electrical power to two of the eight power channels on the station. Flight controllers have been working to route power through the remaining six power channels.”
Additionally, landing plans for the booster’s first stage was shifted from a Return to Launch Site (RTLS) on Landing Zone (LZ)-1 at the Cape to the offshore Autonomous Spaceport Drone Ship (ASDS), “Of Course I Still Love You”, following last weekend’s Crew Dragon anomaly.
Assuming an on-time launch, Dragon will rendezvous and be captured by the station’s 57.7-foot-long (17.6-meter) Canadarm2 early Saturday, 4 May. Handling the robotics from inside the multi-windowed cupola will be Expedition 59 astronauts David Saint-Jacques and Nick Hague. The spacecraft will then be berthed at the Earth-facing (or “nadir”) port of the Harmony node.
Coming barely two weeks after the launch and arrival of Northrop Grumman’s NG-11 Cygnus cargo ship at the adjacent Unity node, the launch and berthing of the CRS-17 Dragon promises a busy time for the six-strong Expedition 59 crew, commanded by Russian cosmonaut Oleg Kononenko, which includes two female astronauts and Canada’s third long-duration ISS resident. This is not the first time that “dual berthed ops” have occurred on station, with the visiting vehicles of both Commercial Resupply Services (CRS) partners, SpaceX and Northrop Grumman, simultaneously in residence. That accolade belongs to the OA-5 Cygnus and CRS-8 Dragon, which arrived in late March and early April 2016. However, the dual berthed ops with NG-11 and CRS-17 in the coming days represents the first occasion that two ships from the two CRS1 providers have arrived in such a short period of time.
“It will definitely be a large workload for the crew, but teams on the ground always make sure there is a plan to keep it manageable before these flights get scheduled,” NASA’s Dan Huot told AmericaSpace. “Cargo ops (both loading and unloading) will happen in tandem with both vehicles.” He added that the entire U.S. Operational Segment (USOS) crew—Canada’s Saint-Jacques and NASA’s Anne McClain, Nick Hague and Christina Koch—“will have cargo loading/unloading tasks on their timeline.”
Aboard Dragon’s pressurized cargo module for CRS-17 will be 3,250 pounds (1,477 kg) of equipment, experiments and supplies for the Expedition 59 and successive crews. This figure includes 1,590 pounds (698 kg) of science hardware, 760 pounds (345 kg) of crew supplies, 754 pounds (342 kg) of vehicle equipment, 165 pounds (75 kg) of computer resources and smaller quantities of Extravehicular Activity (EVA) tools and parts and materials for the station’s Russian Operational Segment (ROS). External hardware in Dragon’s unpressurized “trunk” totals 2,130 pounds (965 kg) and is visibly dominated by NASA’s next-generation Orbiting Carbon Observatory (OCO-3), provided by the Jet Propulsion Laboratory (JPL) in Pasadena, Calif.
Destined for robotic installation on Site 3 of the Exposed Facility (EF) of Japan’s Kibo lab, OCO-3 is expected to spend three years measuring carbon dioxide abundances in the atmosphere, with an accuracy capable of highlighting geographical distributions of sources and sinks on a regional scale. This will contribute greatly to the burgeoning corpus of data about the global carbon cycle and the effect of natural and human processes upon greenhouse gas abundances and distribution.
The first OCO spacecraft was launched atop a Taurus booster back in February 2009, but the failure of its payload fairing to properly separate resulted in the loss of the satellite. A replacement spacecraft, OCO-2, was launched via Delta II in July 2014, with hardware for the fabrication of OCO-3 requisitioned from OCO-2 spares. It carries three parallel, high-resolution spectrometers for simultaneous observation of carbon dioxide and molecular oxygen absorption of sunlight reflected from Earth’s surface at near-infrared wavelengths. This provides OCO investigators with the spectral “fingerprints” of these absorption profiles as part of efforts to determine the numbers of molecules between the upper atmosphere and the surface.
Spares from OCO-2 were utilized to develop OCO-3 for long-duration emplacement upon the ISS exterior. Although similar to its predecessor, it also benefits from the addition of a two-axis pointing mirror for the targeting of cities and other areas on an order of 62 x 62 miles (100 x 100 km) for “snapshot mode” regional mapping. Additionally, OCO-3 includes a 330-foot-resolution (100-meter) context camera and the instrument will be kept cool by means of an on-board cryocooler, which holds the optics at -120 degrees Celsius (-184 Fahrenheit). It is confidently expected that OCO-3 data may be combined with other Earth-resources detectors currently at work on the ISS exterior, notably the Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) and Global Ecosystem Dynamics Investigation (GEDI), which were launched last June and December aboard a pair of SpaceX Dragons.
Yet OCO-3’s evolution has not been a smooth and faultless process. The project was green-lighted for development in December 2015, but was not included in the President’s Proposed Budget for FY2018 when it was released in February 2017. A year later, OCO-3 funding was restored with the Enacted Budget for FY2018, prompting newly-appointed NASA Administrator Jim Bridenstine to emphatically remark that “it’s not been cut…in fact, it’s going to be on-orbit very, very soon”. Thermal vacuum chamber testing of the instrument was completed in May 2018.
The transfer of OCO-3 from the trunk of Dragon—on the nadir face of the USOS—over to the furthest port location on the station will be conducted by means of Canadarm2. “Robotic ops are scheduled to start the day after berthing,” said Mr. Huot, indicating that first motion for OCO-3 can be expected to occur over the weekend, assuming an on-time launch Wednesday, 1 May, and on-time berthing Saturday, 4 May. However, those dates remain uncertain, following Monday’s electrical system issue.
“Entirely ground-commanded robotics operation,” Mr. Huot told AmericaSpace. “Will be ten days total of operations, eight days of transferring cargo to/from Dragon trunk, one day for setup and one day for cleanup.” Also in the trunk is the Space Test Program-Houston-6 (STP-H6), an X-ray communications testbed for space-based demonstration of new technologies for generating beams of modulated X-rays. Future potential applications include more efficient communications for deep-space missions or with hypersonic vehicles where plasma “sheaths” tend to disrupt traditional radio links.
CRS-17 forms part of an extension component to SpaceX’s CRS1 contract with NASA, signed back in December 2008. This initially called for 12 Dragon flights to the ISS, ferrying up to 44,000 pounds (20,000 kg) of payloads and supplies, but three additional missions were secured in early 2015, followed by five more in early 2016. These missions—which will see the CRS1 first round close-out with the CRS-20 Dragon flight—will bridge the gap before the second-round CRS2 contracts gets underway next year.
Coming up in July 2019, the CRS-18 Dragon will ferry the second International Docking Adapter (IDA-3) for Commercial Crew operations, whilst CRS-19 in December will deliver Japan’s Hyperspectral Imager Suite (HISUI) remote-sensing instrument and CRS-20 in spring 2020 will bring the European Space Agency’s (ESA) Bartolomeo commercial payloads anchoring platform uphill for installation onto the forward side of the Columbus lab. When the CRS2 contracted missions commence next year, SpaceX is expected to stage at least six more cargo flights with its upgraded Dragon 2 vehicle through 2024.